(a) Technical Field
The present invention relates to a core-shell cobalt catalyst used for a Fischer-Tropsch synthesis reaction and a method for preparing the same. More particularly, it relates to a cobalt catalyst, which has a core-shell structure including a cobalt-supported and sintered alumina particle as a core and a zeolite powder coated on the surface of the alumina particle to a thickness of 50 μm or greater through mechanical alloying as a shell and is used to prepare hydrocarbons with high octane numbers through a Fischer-Tropsch synthesis reaction, and a method for preparing the same.
(b) Background Art
Fischer-Tropsch (FT) synthesis reaction is a critical process of preparing hydrocarbon compounds from a synthesis gas in gas to liquids (GTL) technology.
In the FT synthesis reaction, long chain n-paraffins are prepared from a synthesis gas (a mixture of H2 and CO gases) through a chain growth reaction. Through the FT synthesis reaction, paraffin-based saturated hydrocarbons with high cetane numbers such as diesel, jet fuel, waxes, etc. are prepared mostly as synthetic fuels. Therefore, an upgrading process is performed to convert them into hydrocarbons with high octane numbers, which are more economical and applied to more areas. That is to say, the synthetic fuels prepared through the FT synthesis reactions are converted to lower aliphatic unsaturated hydrocarbons (light olefins) or isoparaffins through additional upgrading processes such as hydrocracking, isomerization, etc.
Recently, research is underway to develop catalysts for synthesizing specific hydrocarbon compounds directly from the FT synthesis reaction.
U.S. Pat. No. 8,802,741 (patent document 1) discloses preparation of liquid hydrocarbons using a hybrid catalyst obtained by physically mixing a cobalt catalyst component and an acidic zeolite (ZSM-48) component.
Journal of Catalysis 265 (2009), 26-34 (non-patent document 1) discloses a core-shell cobalt catalyst for an FT synthesis reaction of preparing isoparaffins directly from a synthesis gas. Specifically, in the non-patent document 1, a cobalt catalyst having a core-shell structure of [H-β][Co/Al2O3] is prepared by using a cobalt-supported alumina pellet as a core and coating H-β zeolite on the surface of the core to a thickness of 16-36 μm by adding the core to a zeolite precursor solution and conducting hydrothermal synthesis. The non-patent document 1 describes that the core-shell cobalt catalyst exhibits a remarkably improved isoparaffin/n-paraffin molar ratio (Ciso/Cnormal) as compared to a hybrid catalyst prepared by physically mixing cobalt-supported alumina and H-β zeolite.
Langmuir 21 (2005), 1699-1702 (non-patent document 2) discloses a core-shell cobalt catalyst for an FT synthesis reaction of preparing C10 or lower middle isoparaffins from a synthesis gas. Specifically, a cobalt catalyst having a core-shell structure of [ZSM-5][Co/SiO2] is prepared by using cobalt-infiltrated silica as a core and coating ZSM-5 on the surface of the core to a thickness of up to 23.1 μm by adding the core to a zeolite precursor solution and conducting hydrothermal synthesis.
The core-shell cobalt catalysts used for FT synthesis reactions disclosed in the non-patent documents 1 and 2 are prepared by using cobalt-supported alumina or silica as a core and coating zeolite on the outer surface of the core through hydrothermal synthesis. During the hydrothermal synthesis, Si (core)-O—Al (shell) or Al (core)-O—Si (shell) bonding is induced as a sol containing the zeolite precursor reacts with the core and a core-shell structure is formed as a result thereof. The hydrothermal synthesis reaction is performed for a long time of about 7 days at a rotational speed of 10 rpm or lower in order to reduce breakage of the core particle and hardening of the sol. As a result, a shell having a maximum thickness of 36 μm is formed. That is to say, it is impossible to coat a shell to a thickness greater than 36 μm using the hydrothermal synthesis methods disclosed in the non-patent documents 1 and 2.
Through long research experiences, the inventors of the present invention have found out that, when designing a catalyst used for an FT synthesis reaction, the distribution and reducibility of active metal affect the conversion rate of a synthesis gas or the selectivity of a hydrocarbon. Especially, they have realized that, for a cobalt catalyst for FT synthesis which is able to increase the selectivity of hydrocarbons with high octane numbers, uniform distribution of the active metal cobalt and prevention of its reduction are of great importance. In addition, they have found out that spatial restriction is important in synthesis of hydrocarbons with high octane numbers directly from a synthesis gas because hydrocracking and isomerization should occur together with the FT synthesis. That is to say, after the FT synthesis reaction has occurred at the cobalt active metal, if the product is desorbed rather than being adsorbed on the acid site of zeolite, specific hydrocarbon compounds cannot be prepared because hydrocracking and isomerization cannot occur. The sol-gel method or hydrothermal synthesis method presented by the non-patent documents 1 and 2 is limited in synthesis of hydrocarbons with high octane numbers directly from a synthesis gas due to spatial restriction because the coating thickness is small with up to 36 μm.
The inventors of the present invention have attempted to solve the spatial restriction problem by inducing uniform dispersion of cobalt active metal by infiltrating it into a mesoporous alumina support and forming a shell on the outer surface of the active metal-supported alumina by coating a microporous zeolite having many acid sites to a thickness as large as possible. As a result, they have completed the present invention by developing a core-shell cobalt catalyst having a shell thickness of 50 μm or greater by introducing a mechanical alloying process used in the field of powder metallurgy.